This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-325920, filed Oct. 25, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an X-ray CT scanner having a correcting function.
2. Description of the Related Art
An X-ray CT scanner is an apparatus which generates tomogram data by reconstructing, by using a computer, projection data obtained by irradiating an object to be examined with X-rays from the circumference of the object. These X-ray CT scanners are classified into the following three types in accordance with differences between the forms of X-ray beams.
The first one is a xe2x80x9cfan-beam X-ray CT scannerxe2x80x9d which radiates a fan-shaped X-ray beam from an X-ray tube. This fan-beam X-ray CT scanner acquires projection data by using an X-ray detector obtained by arranging about, e.g., 1,000 channels of detecting elements in a line. Projection data acquiring operation is repeated about 1,000 times while the X-ray tube rotates around an object to be examined. This fan-beam X-ray CT scanner is also called a xe2x80x9csingle-slice CT scannerxe2x80x9d because data concerning a single slice are acquired.
The second one is a so-called xe2x80x9cmulti-slice X-ray CT scannerxe2x80x9d in which several X-ray detectors each obtained by arranging about 1,000 channels of detecting elements in a line are juxtaposed in a slice direction. A slightly thick X-ray beam is radiated in accordance with the width of these juxtaposed detectors. This multi-slice X-ray CT scanner is so called because data of several slices can be simultaneously acquired.
The third one is a so-called xe2x80x9ccone-beam X-ray CT scannerxe2x80x9d in which a plurality of detecting elements each composed of a combination of, e.g., a scintillator and a photodiode are two-dimensionally arrayed. A conical or pyramidal X-ray beam is radiated in accordance with the width of these detecting elements in a slice direction. This cone-beam X-ray CT scanner is also called a volume X-ray CT scanner because volume data can be acquired at once.
The research of a cone-beam X-ray CT scanner has been advanced primarily on a system using an image intensifier (I.I.) as an X-ray detector since late 1980s. For example, in xe2x80x9cVolume CT of anthropomorphic using a radiation therapy simulatorxe2x80x9d (Michael D. Silver, Yasuo Saito et al.; SPIE 1651 197-211 (1992)), the results of scan of chest phantoms in an experimental system combining a turntable and an I.I. are discussed. Some cone-beam X-ray CT scanners are beginning to be put into practical use as apparatuses for obtaining the shapes of high-contrast objects such as bones and blood vessels in angiography.
As described above, a cone-beam X-ray CT scanner has a wider divergent angle of an X-ray beam in a slice direction than in the other two types. In other words, the X-ray beam is thick on the rotation central axis. Since this increases the number of paths through which scattered rays reach detecting elements, the scattered ray amount increases. Scattered rays cause abuses, e.g., deteriorate the image contrast. This scattered ray increasing mechanism means that the scattered ray amount varies in accordance with a change in the beam thickness.
An X-ray CT scanner usually performs sensitivity correction in order to equalize the sensitivities of detecting elements. For this purpose, calibration data files (calibration data) for sensitivity correction are acquired by using a phantom (pseudo model). Since scattered rays change in accordance with the beam thickness as described above, these calibration data files must also be selectively used in accordance with the beam thickness.
This paradoxically means that the degree of freedom of beam thickness adjustment is limited by the types of calibration data files that the apparatus has.
Assume, for example, that a calibration data file acquired by a beam thickness X1 and a calibration data file acquired by a beam thickness X2 ( greater than X1) are prepared. In this case, no corresponding calibration data files are prepared for beam thicknesses other than X1 and X2. Therefore, no such beam thicknesses can be set except when the inclusion of a scattered ray error is permitted.
It is an object of the present invention to provide an X-ray CT scanner capable of extending the degree of freedom of setting of an X-ray beam thickness.
According to a certain aspect of the present invention, an X-ray CT scanner comprises an X-ray tube which irradiates an object to be examined with X rays, a variable X-ray limiting device which limits the beam thickness of the X rays, an X-ray detector which detects X rays transmitted through the object and has a plurality of detecting elements arrayed in a matrix manner, a storing unit which stores a plurality of calibration data files corresponding to a plurality of beam thicknesses, a correcting unit which corrects an output from the X-ray detector on the basis of at least one calibration data file read out from the storing unit, a reconstructing unit which reconstructs image data concerning the object on the basis of an output from the correcting unit, and a control unit which controls the variable X-ray limiting device to change the beam thickness of the X rays, independently of the plurality of beam thicknesses to which the plurality of calibration data files stored correspond.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.